This invention relates to the construction of thermal, electrostatic, or other types of print heads powered by electric circuitry.
The print heads of the present invention are very stable, i.e. they are substantially unaffected by changes in temperature and humidity, and do not warp or change in size in any great degree. They are also highly resistant to corrosion. The circuits of the invention are also extremely rugged, resistant to abrasion, and resistant to long-term exposure to adverse environmental effects, such as extremes of temperature or humidity, vibration, salt, and other effects. More importantly, they do not include any organic materials, and therefore do not degrade due to carburization.
The conductors of the print heads of the present invention can be extremely narrow and tightly-packed, yet they can carry relatively high voltages and/or currents. The conductors, and the spaces between them, can be less than about 0.005 inches wide. The invention can therefore be used in the fabrication of ultra-high precision large linear and matrix circuit arrays for thermal, electrostatic, or other electric circuit powered print heads for generating high-resolution images. The invention can also be used to form print heads which produce black-and-white or color halftones.
Various means have been used to generate images on paper and other media. A print head for a thermal printer includes resistive materials placed between circuit paths. The resistive materials are heated and brought in contact with the wax-coated printing paper or ribbon. U.S. Pat. No. 4,604,298 discusses the application of fine-line circuit construction to the fabrication of single-layer thermal print heads.
An electrostatic high-speed printer generates charged sites on a drum or belt, either by an optical scanning technique or by using a linear circuit array disposed near the drum or belt. It has been known to construct such linear circuit arrays using conventional printed circuit technology. These arrays suffer from a very high reject rate and nonlinearity of the pixel array due to the imprecise methodology and materials of construction. They also do not have the operating temperature range, mechanical stability, or ruggedness of this invention.
All of the printer or image-generation devices described above, and other similar electric circuit powered print heads, seek to produce a very uniform and precise pixel format with as many pixels per inch as possible. The human eye is a very good image integrator and can detect extremely small differences in a closely spaced pixel array or closely spaced series of lines. The eye can detect these differences, even though individual pixels or lines cannot be resolved. This is the basic reason that the so-called halftone technology has been so successful for the printing of photographic images with inks.
In the case of thermal printers, a ceramic substrate with a dielectric coating, and a fired-on conductive pattern, comprises the linear array. As mentioned above, U.S. Pat. No. 4,604,298 discusses the application of high-density circuit construction to the manufacture of thermal print heads. But a thermal print head made according to the method of the latter patent has the disadvantage that its conductors are limited in length to about 0.050 inches. Also, the length of the circuit is subject to variation due to the stretching of the screen. Adhesion to the substrate, and uniformity of circuit thickness and width, are highly dependent on the viscosity of the conductive material, the thickness of the mask, and operator technique.
Printing heads are also fabricated from copper-clad circuit board materials, in which case the one makes the circuit board with the print head array lines in the center of the board, with a pixel density one-half that of the finished print head. The board is then fabricated using conventional photolithographic and printed circuit etching techniques. It may also be plated up to increase the circuit thickness or aspect ratio. The board is then sliced in half along its center, and the two halves are placed either face-to-face, with a thin dielectric separator to prevent shorts, or back-to-back. By offsetting the boards, the desired array density is achieved. Arrays having conductor widths of 0.003 inches, with densities of up to 240 lines per inch, have been produced by this general method. These arrays suffer from a very high reject rate and non-linearity of the print head array, due to the imprecise methodology and materials of construction. These circuits also do not have the operating temperature range, mechanical stability, or ruggedness of the present invention.